The invention relates to a method for determining dynamic parameters in a dynamic model of a manipulator. In particular, it relates to dynamic parameters for tool loads and arm loads, so-called load parameters.
By manipulator is meant an industrial robot or external axes connected to the industrial robot, for example for orientation and movement of a work object or for movement of the robot itself. A manipulator comprises one or more arms which are movable in relation to one another, and a handling member which is provided with a tool attachment and which is movable relative to the arm which supports it. The handling member may, for example, be a single platform or a robot hand which is movable in one or more degrees of freedom. The manipulator is provided with a control system which controls the position and orientation of the handling member. For each one of the movement axes of the manipulator, servo equipment with a drive motor is provided. The servo system of each axis is supplied with a reference value for the angle of rotation of the axis and the drive motor of the axis brings the manipulator to move in the axis in question until the axis position corresponds to the reference value supplied to the servo system.
By an axis are meant axis transmissions which may give rise to both rotation and translation of the movable arms and the handling member of the manipulator.
When the manipulator moves, its axes are subjected to forces and torques which originate from dynamic effects such as mass inertia, coupled mass inertia, Coriolis forces, centrifugal forces, the gravitational force, static friction and dynamic friction. In connection with robots, static friction means sliding friction in bearings and gear wheels. This friction depends on the sign of the velocity but not on the magnitude thereof. By dynamic friction is meant that friction which increases with the magnitude of the velocity and which depends on friction in oil, seals, etc. To be able to control the manipulator with high accuracy and maximum velocity, it is necessary that the servo system should compensate for the forces and torques which are caused by the dynamic effects.
How the dynamic effects influence each one of the axes of the manipulator may be described by means of a dynamic model for the manipulator. The dynamic model consists of the movement equation system of the robot. From the dynamic model, the forces and torques which act in each one of the axes may be calculated. A condition for being able to control the movements of the manipulator with high accuracy is that the dynamic effects during the movements may be calculated accurately. The dynamic model comprises model parameters which must be known in order to be able to calculate the dynamic effects. These model parameters comprise mass, centre of gravity and mass inertia for the parts of the manipulator and the load thereof. The dynamic effects from the manipulator parts which are included in the basic design of the manipulator may be calculated accurately since the masses, centres of gravity and mass inertias of these manipulator parts are-well-known.
However, when being used, the manipulator will carry both tool loads and arm loads, and these loads may vary greatly between various applications. Examples of tool loads may be a tool which is mounted on the handling member, for example a glue gun or a welding gun or a workpiece which is moved between two points of the manipulator. Arm loads may, for example, be a transformer which belongs to the tool, a roll with welding wire or a pump for gluing or painting. To obtain high performance, the servo system including the trajectory generator must also compensate for the dynamic effects which arise as a result of tool and arm loads. To be able to calculate the dynamic effects from a load, the model parameters for the load, that is, the mass of the load, the centre of gravity, and the mass inertia must be known. In the following, the model parameters of the load will be referred to as load parameters.
It is, of course, possible to find out the load parameters by weighing the load, measuring its centre of gravity and calculating or measuring its mass inertias and then feeding these values into the control system. This method is known, for example from patent document EP 260 326. The forces and torques which act in each one of the axes are then calculated from the dynamic model, whereafter the servo system compensates for these forces and torques. A disadvantage of this method is that, in, for example, material handling applications, the manipulator may, in the same installation, handle a large number of objects of different weights and shapes, whereby the work of obtaining and feeding the load parameters for all the objects takes a long time and there is a considerable risk of incorrect parameters being fed into the control system.
The publication by Olsen and Bekey, xe2x80x9cIdentification of Robot Dynamicsxe2x80x9d, 1986 IEEE International Conference on Robotics and Automation, pages 1004-1010, discloses a method for identification of the load parameters. This method is based on an equation system of equations of movement being set up for the robot. The equations of movement are parametrized in the mass parameters for the dynamic bodies of the robot. First, the greatly non-linear movement equation system of the robot is linearized. Then, an identification is made by means of the least-squares method. A disadvantage of this method is that also unwanted parameters as, for example, Coulomb friction and viscous friction, must be identified. This implies that errors in the identification of these unwanted parameters directly reduce the accuracy in the identification of the unwanted parameters. Also, unnecessary movements will be required for identifying the unnecessary parameters. For example, the bodies must be moved at a relatively high velocity for accurate identification of the viscous friction. A result of this is that large robot movements are required, which is a disadvantage during load identification. An additional problem which implies that larger movements are required is that noise in measured motor torques and axis angles disturb the identification. A large number of unnecessary calculations will also be made, which is a disadvantage, especially during implementation in a real-time system. Still another problem is that the linearization of the non-linear movement equations leads to identification errors which are difficult to check.
The object of the invention is to suggest an automatic method for identification of load parameters, which is fast and reliable. To avoid the above-mentioned disadvantages, the method shall entail a selective identification of the parameters and not be based on linearization of the movement equations.
What characterizes a method according to the invention will become clear from the appended claims.
The method according to the invention has the following advantages:
the load need not be dismantled during the identification,
the identification is very fast; it takes only a few seconds,
the identification can be carried out anywhere in the operating range of the manipulator,
the identification requires only small movements of the manipulator, so it can be carried out in narrow spaces or at specially reserved locations,
the identification can be carried out with different movement patterns, whereby the movement pattern which suits the current installation is chosen,
the identification can be carried out in the current application program without any special movements having to be made.
The above-mentioned advantages are achieved partly by using a correlation method for identification of the dynamic parameters of axes, partly by making this identification selectively for the gravitation, mass inertia and coupled mass inertia of the axes, and partly by calculating the load parameters exactly from the dynamic parameters of the axes by using the movement equations of the axes. The correlation method is integrating over the whole movement and therefore provides a great noise reduction with respect to the noise of the torque signals and the position signals. By correlating the torque signals with the calculated acceleration signals, the effect of, for example, Coulomb friction, viscous friction, Coriolis forces and centrifugal forces will be suppressed and a selective identification of mass inertia and gravitation be obtained. By finding exact expressions for the relation between the dynamic parameters of the axes and the load parameters, the high accuracy during the identification of the dynamic parameters of the axes will then be maintained when the load parameters are finally calculated.